This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Selective partial oxidation of alcohols (e.g. methanol, ethanol, glycerol, etc.) over heterogeneous catalysts plays an important role in the production of bulk and fine chemicals, as well as in the conversion of biomass-derived compounds to renewable fuels and high value-added products. The correlation between catalyst properties and its performance (reactant conversion and selectivity towards a target product) benefits the development of efficient catalytic processes.
Methanol, the simplest alcohol, is used as a solvent, fuel for specialized vehicles or feedstock for the manufacture of other value-added chemicals, including formaldehyde and olefins. Methanol is considered as the simplest molecule probe candidate, owing to its representative structure, containing C—H, C—O and O—H bonds. About 40% of industrial methanol is converted to formaldehyde, and subsequently into diverse products such as plastics, plywood, paints and fibers. Remaining formaldehyde is consumed in the manufacture of textiles, paper, fertilizers and miscellaneous resinous products. Formaldehyde is produced commercially by catalytic selective oxidation of methanol. The most common processes of converting methanol to formaldehyde utilize supported silver (Ag) or molybdenum-iron (Mo—Fe) catalysts. The silver process operates at atmospheric pressure and the reaction temperature is in the range 560-600° C. Under these conditions, methanol conversion is typically 65-75%, while formaldehyde selectivity is about 90%. In the Mo—Fe process, an excess of air is used to ensure nearly 100% methanol conversion and avoid the explosive limits of methanol (6.7-36.5 vol. % in air). The reaction temperature is lower than that for the silver process, but yet in the range 250-400° C. The formaldehyde yield (methanol conversion times formaldehyde selectivity) is improved to 95%, with methanol conversion as high as 98-99%. Other catalyst candidates for methanol selective oxidation to formaldehyde have also been reported in the literature, including VOx (˜400° C.), Cr—Mo (˜300° C.), Fe—Cr—Mo (˜300-360° C.), Mo—V—Cr—Bi—Si (˜425° C.), MoO3 (˜300-350° C.) and Bi2O3—MoO3 (280° C.). In all these cases, however, relatively high temperature (250-600° C.) is always used, leading to high energy and operating costs as well as large capital investment.
Using noble metals such as platinum (Pt) or pladium (Pd), low-temperature (<˜120° C.) oxidation of methanol has been investigated previously. Owing to the superior activity over noble metals, however, formaldehyde production is limited using these catalysts because over-oxidized products (e.g. formic acid and CO2) are typically generated. Using controlled Pd nano-particles with specific metal particle size as catalyst, the formaldehyde selectivity was only 20-30%. The use of Pt-based catalysts for formaldehyde production from methanol oxidation have also been reported. These, however, result in either low methanol conversion (<2.5%), or low selectivity towards target formaldehyde product. Recently, rather than over-oxidized products and formaldehyde, methyl formate was selectively (with selectivity>95%) synthesized by methanol coupling over Au catalysts. Similar high selectivity values towards methyl formate were also achieved over Pd-based catalysts.
In general, reducible oxides (CeO2, TiO2, V2O5, Bi2O3, etc.) can achieve high selectivity for certain non-over-oxidized products. Among these reducible oxide candidates, bismuth oxides show flexible oxidation states, including +1, +2, +3, +4 and +5, and typically +3. This suggests that Bi is a potentially good catalytic promoter for selective oxidation. When reducible oxides are used as promoters, in addition to noble metals, noble metal/reducible oxide interfaces are formed, which are able to tune selectivity towards target products by providing surface oxygen vacancy. In our prior works, some applications of bimetallic Pt—Bi catalysts were reported, e.g. glycerol conversion to 1,3-dihydroxyacetone (DHA), and guaiacol deoxygenation by the use of methane as reductant. Using density functional theory (DFT) in our recent publication, we concluded that the BiOx species is formed in situ at the interface of the originally reduced Pt—Bi bimetallic catalyst. A cooperative effect between Pt as the primary component and BiOx as the promoter was further identified for DHA formation from glycerol oxidation. Thus the Pt—BiOx interface favors O—H, rather than C—H, bond breaking. There is currently no available method which converts methanol to formaldehyde with high selectivity (e.g. >90%) at temperature<120° C.
Thus there is an unmet need for method which converts methanol to formaldehyde with high selectivity (e.g. >90%) at temperature<120° C.